1. Field of the Invention
The present invention relates to a microtip, focusing gate and high microtip density electron source. It also relates to a flat screen using such a source.
2. Discussion of the Background
The documents FR-A-2 593 953 and FR-A-2 623 013 describe field emission-excited cathodoluminescence display devices. These devices comprise a microtip emitting cathode electron source.
As an illustration, FIG. 1 is a transversal section view of such a microtip display screen. For simplification purposes, only a few aligned microtips have been represented. The screen is composed of a cathode 1, which is a plane structure, positioned opposite another plane structure forming the anode 2. The cathode 1 and the anode 2 are separated by a space in which a vacuum is produced. The cathode 1 comprises a glass substrate 11 on which the conductive level 12 in contact with the electron emitting tips 13 is deposited. The conductive level 12 is coated with an insulating layer 14, e.g. silica, which is itself coated with a conductive layer 15. Holes 18, approximately 1.3 xcexcm in diameter, have been produced through the layers 14 and 15 up to the conductive level 12 to deposit the tips 13 on said conductive level. The conductive layer 15 is used as an extraction gate for the electrons emitted by the tips 13. The anode 2 comprises a transparent substrate 21 coated with a transparent electrode 22 on which luminescent phosphors or luminophors 23 are deposited.
The operation of this screen is described below. The anode 2 is brought to a positive voltage of several hundred volts with reference to the tips 13 (typically 200 to 500 V). On the extraction gate 15, a positive voltage of several tens of volts (typically 60 to 100V) with reference to the tips 13 is applied. Electrons are then extracted at the tips 13 and are attracted by the anode 2. The electrons"" paths are comprised in a top half-angle cone xcex8 depending on different parameters, including the shape of the tips 13. This angle induces a defocusing of electron beam 31 which increases with the distance between the anode and the cathode. However, one of the ways to increase the efficiency of phosphors, and therefore the screen brightness, is to work with higher anode-cathode voltages (between 1000 and 10,000 V), which implies increasing the distance between the anode and the cathode further to prevent the formation of an electric arc between the two electrodes.
In order to retain a good resolution on the anode, the electron beam must be refocused. This refocusing is obtained conventionally using a gate that can be placed between the anode and the cathode or positioned on the cathode.
FIG. 2 illustrates the case in which the focusing gate is positioned on the cathode. FIG. 2 takes the same example as FIG. 1 but limited to a single microtip for more clarity in the drawing. An insulating layer 16 has been deposited on the extraction gate 15 and supports a metal layer 17 used as a focusing gate. Holes 19, of suitable diameter (typically between 8 and 10 mm) and concentric with the holes 18, have been engraved in the layers 16 and 17. The insulating layer 16 is used to insulate the extraction gate 15 and the focusing gate 17 electrically. The focusing gate is polarised with reference to the cathode so as to give the electron beam 32 the form represented in FIG. 2.
In the case of a microtip screen without a focusing gate, such as that shown in FIG. 1, the distance between two adjacent microtips is of the order of 3 xcexcm. For a microtip screen with a focusing gate, as represented in FIG. 2, this distance is of the order of 10 to 12 xcexcm. In this case, the microtip density, i.e. the electron emitter density, is between 9 and 16 times lower. This results in a decrease in screen brightness.
In a flat screen, the luminophors are deposited on the anode in the form of parallel bands, which are successively red-green-blue, etc. For a good restored image quality, the colours must not be mixed. For this, all the electrons emitted by a pixel of a given colour must go to the corresponding luminophor and not to the adjacent luminophors. This result is obtained by the focusing phenomenon. Given the band structure of the luminophors, it is important that the focusing is carried out in the direction perpendicular to these bands to prevent mixing of colours.
The invention makes it possible to remedy the problem of low microtip density posed by prior art focusing gate electron sources. This is obtained by replacing the circular apertures of the focusing gate by slits.
The invention proves to be particularly effective when applied to flat screens in which the luminophors are arranged in bands. It is proposed to etch, in the focusing gate, apertures in the form of slits, with the microtips aligned on the axes of these slits. By arranging the luminophors located on the anode in the form of bands parallel to the electron source slits and just above the corresponding slits, the electrons emitted by the microtips of these slits remain concentrated on the luminophor band facing them. Therefore, there will be no mixing of colours. If the focusing is not obtained in the direction of the bands, a slight spreading of the pixel in this direction is produced, which has a relatively insignificant effect on the image quality.
Therefore, the focusing gate according to the present invention performs a focusing function in a single direction.
Therefore, the invention relates to a microtip electron source comprising:
at least one electron emission zone composed of a plurality of microtips connected electrically to a cathode conductor,
at least one gate electrode, positioned opposite said electron emission zone and pierced with apertures located opposite the microtips, to extract the electrons from the microtips,
an emitted electron focusing gate positioned opposite the gate electrode, and equipped with aperture means comprising at least one slit located opposite at least two successive microtips,
characterised in that the focusing gate is separated from the extraction gate electrode positioned opposite it by a layer of electrically insulating material with a slit aligned with the focusing gate slit, or a succession of holes aligned with the focusing gate slit, of a width less than that of the focusing gate slit.
According to an advantageous arrangement, the microtip electron source may comprise a plurality of electron emission zones arranged in the form of a matrix in rows and columns, with the number of cathode conductors and gate electrodes corresponding to the rows and columns to give the microtip electron source a matric access.
If each emission zone comprises several rows of microtips, each row of microtips has one or more corresponding slits in the focusing gate.
The invention also relates to a device comprising a first and second plane structure maintained opposite and at a determined distance from each other by means forming a spacer, the first plane structure comprising, on its inner device face, a microtip electron source such as that defined above, and the second plane structure comprising, on its inner device face, means forming the anode.
Such a device may be used to form a flat display screen, with luminophors placed between the microtip electron source and the means forming the anode.
The invention also relates to a flat display screen comprising a first and second plane structure maintained opposite and at a determined distance from each other by means forming a spacer, the first plane structure comprising, on its inner screen face, a microtip electron source such as that defined above, in which each emission zone comprises several rows of microtips and each row of microtips has one or more corresponding slits in the focusing gate, and the second plane structure comprising, on its inner screen face, means forming the anode, a conductive layer forming the anode and supporting luminophors arranged in alternating red, green and blue bands, with each band located parallel to and opposite a series (row or column) of electron emission zones, with the main axis of the focusing gate slits directed in the direction of the luminophor bands and each emission zone defining a pixel for the display screen.
Naturally, the microtip electron source according to the present invention may be used in relation with anodes of different structures, particularly conventional structures produced for cathode ray tube screens, adapted for flat screens.
The invention also relates to a microtip and focusing gate electron source manufacturing process, comprising:
a step in which the following are successively deposited on one face of an electrically insulating substrate: cathode connection means, a first electrically insulating layer of a thickness adapted to the height of the future microtips, a first conductive layer intended to form the extraction gate, a second electrically insulating layer of a thickness corresponding to the distance to separate the extraction gate from the focusing gate,
a step consisting of piercing the second insulating layer with holes up to the first conductive layer, with the axes of the holes corresponding to the axes of the future microtips and the diameter of these holes adapted to the size of the future microtips,
an electrolytic deposition step of conductive material in said holes, with the first conductive layer acting as the electrode during the electrolysis, the electrolytic deposit filling said holes from the first conductive layer and flowing onto the second insulating layer, first of all giving the electrolytically deposited conductive material the shape of mushrooms, the caps of which rest on the second insulating layer, with the electrolytic deposit subsequently producing, due to coalescence of the mushroom caps formed in adjacent and sufficiently close holes, an approximately semi-cylindrical shaped mass for each set of adjacent and sufficiently close holes,
a deposition step of a second conductive layer intended to form the focusing gate, with the material of this second conductive layer being different to that of the electrolytically deposited conductive material,
an electrolytically deposited material removal step, with this removal leaving, in the second conductive layer, one slit for each previously formed mass, the main axis of which is aligned with the holes with which it was formed,
a hole deepening step up to the cathode connection means,
an etching step of the second insulating layer to reveal the first conductive layer,
a microtip formation step on the cathode connection means revealed by the hole deepening step.
The hole deepening step may be performed by etching. This step and the second insulating layer etching step may be performed simultaneously.
The invention also relates to a microtip and focusing gate electron source manufacturing process, comprising:
a step in which the following are successively deposited on one face of an electrically insulating substrate: cathode connection means, a first electrically insulating layer of a thickness adapted to the height of the future microtips, a first conductive layer intended to form the extraction gate, a second electrically insulating layer of a thickness corresponding to the distance to separate the extraction gate from the focusing gate, a masking layer,
a step consisting of piercing holes through the complex formed by the masking layer, the second insulating layer and the first conductive layer up to the first insulating layer, with the axes of the holes corresponding to the axes of the future microtips and the diameter of these holes adapted to the size of the future microtips,
a hole deepening step in the first insulating layer up to the cathode connection means,
a lateral etching step of the second insulating layer to increase the diameter of the holes pierced previously to a determined value, with this lateral etching being able to render adjacent and sufficiently close holes secant,
a masking layer removal step,
an electrolytic deposition step of conductive material in said holes, with the first conductive layer acting as the electrode during the electrolysis, the electrolytic deposit filling said holes from the first conductive layer and flowing onto the second insulating layer, first of all giving the electrolytically deposited conductive material the shape of mushrooms, the caps of which rest on the second insulating layer, with the electrolytic deposit subsequently producing, due to coalescence of the mushroom caps formed in adjacent and sufficiently close holes, an approximately semi-cylindrical shaped mass for each set of adjacent and sufficiently close holes,
a deposition step of a second conductive layer intended to form the focusing gate, with the material of this second conductive layer being different to that of the electrolytically deposited conductive material,
an electrolytically deposited material removal step, with this removal leaving, in the second conductive layer, one slit for each previously formed mass, the main axis of which is aligned with the holes with which it was formed,
a microtip formation step on the cathode connection means through the holes produced in the first conductive layer and the first insulating layer.
The hole deepening step in the first insulating layer and the lateral etching step of the second insulating layer may be performed simultaneously by isotropic etching.
Irrespective of the process implemented, the step consisting of piercing holes may be carried out by etching. The electrolytically deposited material removal step may be carried out by chemical dissolution. The cathode connection means may be obtained by deposition of cathode conductors on the substrate, followed by deposition of a resistive layer.